Long-chain alcohol induced phase transition in lyotropic mixed polyoxyethylene-type surfactant liquid-crystals

The structural change and phase transition in the mixed surfactant liquid crystalline (LC) system of nonaethyleneglycol dodecylether (C12EO9), polyoxyethylene (20) sorbitan monostearate (Tween 60) and water has been studied on addition of primary alcohols CnH2n+1OH (CnOH, n = 10, 12, and 14) or replacement of C12EO9 with CnOH. In the alcohol-added systems, the C12EO9/Tween 60/CnOH/H2O mixture at a 1:1:x:60 molar ratio for n = 12 undergoes hexagonal to layered phase transition just beyond the critical composition of x = 0.5, whereas the phase transitions for n = 10 and 14 occur at lower compositions. In the alcohol-replaced C12EO9/Tween 60/CnOH/H2O systems at a 1 − y:1:y:60 molar ratio, the hexagonal LC structures for n = 10 and 12 are kept up to y = 0.5 and 0.7, respectively, although the hexagonal phase for n = 14 is transformed into a layered structure beyond y = 0.3. The y = 0.3 composition for n = 12 affords a very highly ordered hexagonal LC, even compared with the alcohol-free phase. The rod-to-rod distance for the hexagonal LCs consisting of cylindrical rodlike micelles in the alcohol-replaced systems increases remarkably with an increase of y, in contrast to a slight increase for the same parameter in the alcohol-added systems. The phase transition mechanism is also discussed.